Hi Cristina,
It seems that you have set the black color for you site and our menu icon is in transparent image. So menu icon disappeared.
you have to change the menu bar color to any light color expect dark black.
To change the menu bar color Go to Appearance -> Theme Options -> Design Options Tab -> Custom CSS paste the following CSS code and Click on save all changes button. I have set the white color below. You can change it to any of your choice.
#access {
background-color: #fff;
}
Thank you!